In recent years, in consideration of a global environment, a reduction in carbon dioxide (CO2) emissions has been highly desired. Therefore, enhancements of a lean burn operation are being made in order to achieve better fuel consumption of an internal combustion engine of a vehicle.
Exhaust gas emitted from a lean-burn gasoline engine, a direct-injection engine or a diesel engine contains a lot of oxygen. Thus, a conventional three-way catalyst can hardly reduce and purify nitrogen oxide (NOx). Further, when particulate matter is discharged, a common monolithic catalyst cannot purify the particulate matter. Particularly, in order to sufficiently purify exhaust gas emitted from the diesel engine, not only NOx but also particulate matter (PM) are necessarily purified. Therefore, advances are being made in the development for purification of exhaust gas in various ways.
One of the effective methods for purifying PM is to trap PM emitted from an internal combustion engine using a filter. In this case, since a pressure loss increases as the amount of the PM deposited on the filter increases, the filter is necessarily heated up to burn and remove the PM. In the present circumstances, the filter is required to be heated to at least 600° C. or more so as to remove the deposited PM quickly to regenerate the filter. In order to further increase a burning rate of the PM to regenerate the filter for a short time, the filter is necessarily heated to 650° C. or more.
In this case, a large amount of fuel is supplied to the internal combustion engine, so as to increase an exhaust gas temperature. In addition, unburned fuel is discharged from the internal combustion engine to be burned by an oxidation catalyst provided in the front portion of the filter, so as to increase the exhaust gas temperature. However, such an operation causes a decrease in fuel consumption. Moreover, the deposited PM is burned in a self chain reaction at such a high temperature. Therefore, thermal runaway is caused when the PM is excessively deposited, and as a result, the filter may be damaged.
In order to burn PM deposited on a filter effectively at a lower temperature, various ways to support a catalyst component on a filter have been attempted (for example, refer to Patent Literature 1).
However, the catalyst component supported on the filter is solid, and the PM to be removed is also solid. Since a rate of contact between the solid substances is low, it is difficult to obtain a sufficient effect of reaction promotion. Therefore, for example, a suggestion to promote PM burning by causing a concentration gradient of the catalyst component to be supported has been proposed (for example, refer to Patent Literature 2).
When an excessively large amount of a catalyst is supported on a filter in order to promote PM burning, pores of the filter are blocked, and a pressure loss is thus increased. Thus, a suggestion to adjust a porosity, a pore diameter and the catalyst coated amount of the filter has been also proposed (for example, refer to Patent Literature 3).
In addition, a suggestion to prevent thermal runaway when a large amount of PM is deposited on a filter has been proposed. For example, a suggestion to control the introduction amount of air when PM is burned has been proposed (for example, refer to Patent Literature 4).
Moreover, researches for materials capable of starting burning PM at a lower temperature have been advanced. For example, various types of components are added based on cerium, and the amount of the components to be added is controlled, so as to enhance low temperature activation (for example, refer to Patent Literature 5).